1. Technical Field of the Invention
The present invention relates to a multi-chamber fluidized bed classifying apparatus in which a raw material having a wide particle size distribution, such as coal, slug, etc., are dried, heated or cooled and which classifies the raw material into fines (powder) and grains.
2. Description of the Related Art
As the conventional fluidized bed classifying apparatus, there is an apparatus disclosed in Japanese Unexamined Patent Publication (kokai) No. 6-343927. In the apparatus, it is performed to control the size of particles to be classified (corresponding to "flowing velocity in free board) by adjusting the flowing velocity of fluidization gas forming a fluidized bed. In this way, the material is separated into the grains remaining in the fluidized bed and the fines scattering from the bed into the free board. In the same apparatus, thereafter, the grains is withdrawn from the fluidized bed, while the exhaust gas containing the fines is extracted out of the free-board to collect the fines by means of a cyclone or the like.
In the Publication No. 6-343927, it is described that an auxiliary gas for classification is introduced into an discharge chute for discharging the grains from the fluidized bed, thereby preventing the fines having a particle size smaller than a predetermined particle size from entering into the discharge chute. In the same publication, it is also described that the inside temperature of the fluidized bed is monitored and thereupon, the fluidization gas forming the fluidized bed is heated so that the so-monitored temperature reaches a specified temperature necessary to dry the raw material.
As an example of the apparatus in which its interior having a fluidized bed is divided into plural chambers, there is known a fluidized bed drying apparatus shown in Japanese Unexamined Utility Model Publication (kokai) No. 2-48785. In the apparatus, a fluidizing chamber having the fluidized bed is divided into plural chambers by a partition plate while remaining a clearance against a gas diffusing plate. On the downstream side of the partition plate in the flowing direction of the fluidized bed, a dam plate is arranged in parallel with the partition plate. Between the partition plate and the dam plate, gas is spouted upward in order to establish both uniform residence time of the dried material and uniform dryness thereof.
In case of processing the raw material (e.g. coal, slug, etc.) in the fluidized bed, there exist coarse particles which are not fluidized due to their wide particle size distribution despite that the fluidized bed is defined by spouting the fluidization gas from the downside of the gas distributing plate.
In order to transfer the coarse particles which are not fluidized, Japanese Unexamined Patent Publication (kokai) No. 5-71875 discloses a fluidized bed apparatus which ejects the gas obliquely upward along an inclined face of the gas distributing plate so that the grains can jump over a jumping table.
Additionally, Japanese Unexamined Patent Publication (kokai) No. 6-281110 also discloses a large lump discharge apparatus for discharging large lumps from the fluidized bed. In the apparatus, a gas distributing plate is arranged in a fluidized bed furnace. At the intermediate position of the gas distributing plate, the fluidized bed furnace is provided, on a bottom part thereof, with a recess. Further, a large lump discharge chute is arranged so as to penetrate a wind box. The upper end of the large lump discharge chute is inserted into the recess, for discharging large lumps from the apparatus.
For the conventional built-in gas distributing plate in the fluidized bed apparatus, there are generally known a gas distributing plate in the form of a cap and a perforated gas distributing plate having numerous gas ejection holes.
Japanese Unexamined Patent Publication (kokai) No. 6-287043 discloses a cement-clinker burning apparatus which includes a fluidized-bed burning furnace arranged below a gas distributing plate in a fluidized-bed granulating furnace. In operation, the granulated material is thrown into the fluidized bed burning furnace via a discharge gate facing the fluidized bed in the fluidized bed granulating furnace. This cement-clinker burning apparatus is provided with ventilating means for blowing gas from the discharge gate into the fluidized-bed granulating furnace. The apparatus further includes a classifying gate constructed to enter into or leave out the discharge gate through the lateral side of the furnace body. Owing to the provision of the classifying gate, the opening area of the discharge gate can be adjusted larger or smaller to extract the fines out of the particles falling from the discharge gate.
In the fluidized bed classifying apparatus of Japanese Unexamined Patent Publication (kokai) No. 6-343927, it is executed to control a gas quantity of the fluidization gas in order to regulate the classified particle size. However, there actually exists a gas-quantity adjustable range necessary for maintaining the fine and stable fluidized bed. Therefore, there is a case that it is impossible to control the fluidization gas so as to have both gas quantity and temperature necessary for the drying process or the like, while maintaining the proper fluidized bed without influencing the apparatus'classifying performance. Additionally, by only supplying the classification auxiliary gas into the discharge chute for grains, it is impossible to obtain a sufficient secondary classifying effect to separate the fines (powder) finer than the classified particle size. Further, if it is necessary to exchange the perforated gas distributing plate subjected to wear, corrosion, etc., then enormous time and cost would be required. If the raw material contains many large lumps because of its wide particle size distribution, the large lumps may stay right under a material loading part of the apparatus, causing a standstill of fluidization.
The conventionally-known gas distributing plate of cap type is not suitable to treat the grains of a wide particle size distribution because there is a large "particle" dead area causing the large lumps to be remained immovably. Additionally, the same plate has problems of worn caps and blocked nozzles. On the contrary, the perforated gas diffusing plate is capable of full fluidization in spite of somewhat large lumps in the raw material, on condition that the plate is appropriately designed on consideration of the uniformity of spouting, the immovable part of particles among the nozzles, jet height, etc. Therefore, the perforated gas diffusing plate is superior to the former plate against wear and blocking. Nevertheless, there is a fault that a relatively great amount of processed articles fall from the plate, causing the falls to be stacked in the wind box.
In the multi-chamber fluidized bed drying apparatus as shown in Japanese Unexamined Utility Model Publication (kokai) No. 2-48785, if only the particle size of material to be dried is generally uniform, then both uniform residence time and uniform dryness are accomplished to some extent. Nevertheless, when processing the powder and granular material having a wide particle size distribution, it is impossible to discharge the processed material to the downstream side of the apparatus since the coarse particles and the large lumps are stacked underside the partition plate.
The apparatus described in Japanese Unexamined Patent Publication (kokai) No. 5-71875 requires spouting the gas at a considerably high speed. Consequently, the gas distributing plate is easy to be worn because of its high pressure loss, requiring enormous time and cost for the replacement of the worn plate with a new one. Furthermore, since the gas distributing plate is complicated in structure, the maintenance is troublesome and complicated. Also, since the maximum transferable particle size depends on the spouting speed of gas, there is a case of large lumps staying on the gas distributing plate, so that the apparatus may be standstill in operation. In order to transfer the grains certainly, it is necessary to increase the flowing speed of the fluidized bed, causing the scattering quantity of fines to be increased.
In the apparatus described in Japanese Unexamined Patent Publication (kokai) No. 6-281110 where the large lumps are discharged through the gas distributing plate and the center of the wind box, the apparatus is complicated in structure. Moreover, it is impossible to discharge the large lumps certainly and finally, the large lumps are gradually stacked as time passes, so that the fluidization itself of the fluidized bed is deteriorated.
In the system described in Japanese Unexamined Patent Publication (kokai) No. 6-287043 where the classifying gate is arranged on the bottom of the fluidized-bed granulating furnace, the particles are classified and discharged while being drifted in the gas current on the bottom of the fluidized-bed granulating furnace. Thus, using the classification gas at a small flowing speed in classifying the fines, the particles enter into the classifying sections in the chute once and for all, so that the respective sections are filled up with the particles. Therefore, it is impossible to exhibit the classification effect sufficiently.